The present invention relates to the manufacture of plate fin and tube heat exchangers. Specifically, the invention relates to a method and apparatus for controlling the operation of a tube expander used in one step of the manufacturing process.
In manufacturing a typical plate fin and tube heat exchanger, such as may be used in an air conditioning or refrigeration system or in an engine cooling system, U-shaped or hairpin tubes are inserted into holes in the fins and tubesheets of the heat exchanger until the open ends of the hairpin tubes protrude beyond one of the tubesheets. The walls of the tubes are then expanded radially, using a tube expander, to make firm contact between the fins and the tubes and tubesheets to ensure good heat transfer and structural integrity. The open ends of the hairpin tube legs are also expanded radially to a greater diameter than the remainder of the tube to form a bell or socket. Short U-shaped tubes, or return bends, are then inserted into the belled ends and secured by a suitable process such as welding, brazing or soldering to form a closed fluid flow path within the heat exchanger. U.S. Pat. No. 4,228,573 provides a general description of the entire process of manufacturing plate fin and tube heat exchangers according to one method. U.S. Pat. Nos. 4,850,101 and 4,858,305 provide descriptions of two different methods of manufacturing plate fin and tube heat exchangers incorporating tension tube expanders.
The tension tube expansion process results in an overall decrease in the length of the tube being expanded. It is therefore desirable to expand the two legs of each hairpin at the same time. If only one leg of the hairpin is expanded in a given expansion operation, only that leg will be decreased in length and therefore the end of the other, unrestrained leg will be drawn out of the tubesheet. The increased protrusion of the unexpanded leg can hamper or complicate the expansion of that leg in a subsequent operation.
In some tube expansion methods, including most employing compression expansion, one of which is described in U.S. Pat. No. 4,228,573, and in some employing tension expansion, one of which is described in U.S. Pat. No. 4,584,765, all the hairpin tubes in the heat exchanger are expanded at the same time. Thus both legs of each hairpin tube are expanded simultaneously and the uneven tube protrusion problem does not arise.
In other methods of expansion, such are described in U.S. Pat. Nos. 4,850,101 and 4,858,305, however, something less than all hairpin tubes are expanded in a given expander stroke, with multiple strokes required to expand all the tubes in the heat exchanger. In tube expansion methods of this type, it is necessary that the expander be properly positioned and sequenced to expand both legs of a given hairpin at the same time.
Fluid flow and heat transfer considerations result in various heat exchanger designs having various hairpin tube configurations, complicating the task of hairpin tube leg pair identification. One method sometimes used to identify hairpin tube leg pairs for expansion in the same operation is to provide an operator with a diagram of the tube arrangement in the heat exchanger as an aid in manually positioning the heat exchanger with respect to the expander. This method, of course, is slow, labor intensive and not economical in large scale manufacturing operations. Another method is to use some type of programmable machine controller to control the positioning of the expander with respect to the heat exchanger. U.S. Pat. Nos. 4,850,101 and 4,858,305 describe this method. Programmed control of the expander has limitations, particularly when the size of the heat exchangers being manufactured or the complexity of the tube arrangements increases or if it is desired to use the same expander to manufacture more than one type of heat exchanger. Each different model of heat exchanger expanded would require a separate program and program size would increase as heat exchanger size and complexity increased. In addition, there must be some means provided for matching the control program to the specific design of heat exchanger being expanded and to verify that the expander is properly indexed to the heat exchanger at the start of an expansion operation. Moreover, the programmed controller is " blind" to programming, setup and indexing errors, leading to the possibility of manufacturing defects until the error or errors are discovered by other means, such as inspection, and corrected.